linux-kernel - Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

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Message-ID: <20190213144145.GY32494@hirez.programming.kicks-ass.net>
Date:   Wed, 13 Feb 2019 15:41:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     Will Deacon <will.deacon@....com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        mingo@...hat.com, catalin.marinas@....com, james.morse@....com,
        hpa@...or.com, valentin.schneider@....com
Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called
 in unsafe region

On Wed, Feb 13, 2019 at 02:39:22PM +0000, Julien Thierry wrote:
> Hi Peter,
> 
> On 13/02/2019 14:25, Peter Zijlstra wrote:
> > On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:
> >> The difference is because getting preempted in the sequence above is
> >> triggered off the back of an interrupt. On arm64, and I think also on x86,
> >> the user access state (SMAP or PAN) is saved and restored across exceptions
> >> but not across context switch.
> > 
> > A quick reading of the SDM seems to suggest the SMAP state is part of
> > EFLAGS, which is context switched just fine AFAIK.
> > 
> I fail to see where this is happening when looking at the switch_to()
> logic in x86_64.

Yeah, me too.. we obviously preserve EFLAGS for user context, but for
kernel-kernel switches we do not seem to preserve it :-(